1. Field of the Invention
This invention relates in one embodiment to a high efficiency monolithic multijunction solar cell comprising lattice-mismatched materials, and a method for making the same.
2. Description of the Related Art
To date, the highest conversion efficiency achieved for monolithically-grown, series-connected multijunction solar cells has been obtained using semiconductors with atomic lattice spacing closely matched to that of the growth substrate. In particular, the highest conversion efficiency multijunction solar cells have been formed using Group III-V semiconductors with appropriately sequenced band gaps, grown monolithically with atomic lattice spacing closely matched to that of a substrate, either GaAs or Ge. The semiconductors and band gaps used to date include Ga(x)In(1−x)P, where x is about 0.5, for a band gap range of about 1.85 to 1.9 eV for the top subcell (the subcell on the front surface of the cell); InGaAs, where the In content is low, giving a band gap range about 1.38 to 1.43 eV for the mid subcell; and Ge, with a band gap of about 0.66 eV for the lower subcell.
Theoretical calculations show that if the Ge subcell could be replaced with a subcell using material with band gap in the range of about 0.95 to 1.05 eV, higher efficiencies are attainable because of more effective use of the incident solar spectrum. Attempts to grow lower subcells in this band gap range on a GaAs or Ge substrate have not been successful, because the lattice-mismatch, about 2% with compounds like In(x)Ga(1−x)As where x˜0.3, reduced the quality and performance of the lower subcell. The lattice mismatch also degraded the performance of the upper subcells even if attempts were made to compensate for the lattice mismatch.
Several schemes have been used to minimize the lattice-mismatch, including the growth of lower subcells comprising in(x)Ga(1−x)As layers with x graded from 0 to about 0.3, or by varying temperature schedules during growth of the lower and upper subcells to reduce the mechanical strains caused by the mismatch. Inclusion of additional elements such as nitrogen or boron in the InGaAs in the lower subcell maintained the band gap and changed the lattice spacing to match the substrate, but the added elements led to very poor electronic quality. The decreased current density in the series-connected subcells severely limited the cell performance, even if the upper subcells had good performance.
What is needed are methods for dealing with the lattice mismatch of lower subcells and improving the efficiency of solar cells.